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  1. #1

    Default What does "detonation" sound like? Engine "knock"?

    So I'm afraid I either have some detonation or "knock" in my new engine in my 89 2500 burb 350 TBI.

    Sometimes...usually shortly after I start the truck when I'm accelerating up an incline (where more torque is on the engine) I get a sound that sounds like my radiator fan is whacking into my fan shroud. I thought the extra torque was "tilting" the engine and the fan was running into something.

    But when I had my finace sit in the truck with and hit the gas and the brakes at the same to time to make the noise...the noise happened, but the fan wasn't hitting anything! It sounded more like it was coming from the engine. Only happens under load, starting from a stop un a hill etc.

    The other thing I noticed was it seems to go away once the truck warmed up.

    This makes me nervous. I'm not sure why my engine would be detonating unless the distributor is shot or was slipping??? But what is "engine" or "rod" knock....I've heard it used all the time but I guess I've never stopped to know for sure what it is?

    Am I experiencing detonation? Or rod knock? What should I do about it?

  2. #2


    Can you elaborate on the "new engine" I'm not sure I can describe pinging.
    When you hear hoofbeats, look for horses not zebras.

  3. #3
    Legend tlperry68's Avatar
    Join Date
    Oct 2008
    Huntington Beach, CA


    The only detontions I've done sounded like KAAABOOOOOOOOOOM. They weren't on motors.

    Rod knock is virtually alway pesent. If you only hear it s the RPM increases you might want to check the motor mounts. Sounds like something is moving and causing an impact into another moving part.

    How new is the motor?
    Trevor - Huntington Beach, CA
    2007 GMC 2500 4X4

  4. #4


    If you have heard a Mercedes diesel engine at a stop light or a big truck diesel engine, that is the sound of knocking. Or a United Parcel Service Truck. I know when they are here from the sound of the truck!

    This can be caused by the timing being too advanced. The timing is all done by the engine computer. In general, sensors which input information to the computer which can change the timing are for the following...

    Crankshaft position
    Engine speed (RPM)
    Engine load (manifold pressure or vacuum)
    Atmospheric (barometric) pressure
    Engine coolant temperature
    Intake air temperature (retards timing when hot)

    (Your specific vehicle may not have sensors for all the above)

    Then also there should be a knock sensor. When the engine computer detects knocking via this sensor, it should retard the timing, then slowly advance it to the best setting without knocking. So possible problem with knock sensor not working too.

    Then there can be other problems which cause knocking. Older vehicles have this problem...

    A GM Factory Service Manual Set would give instructions for checking each of the above sensors. (Order from or a GM dealer.)

    Or take it to a GM dealer and they could put it on their fancy electronic testing gizmo's and tell what is going on.
    Last edited by bill190; 07-20-2009 at 05:19 PM.
    92 GMC Sierra 2500

  5. #5


    You have to set the timing on the 89 TBI. I would start there first. Get a Chiltons or Haynes to get the procedure. He has a knock sensor, but if the distributor is way off, it may not be able to correct.

  6. #6


    Thanks guys,

    I did set the timing the proper way (disconnect the timing computer wire etc) when I installed the engine (just a rebuilt stock TBI 350 motor) so it was at least o.k. to begin with. So if this is pinging from too advanced ignition timing then either the distributor was not tightened down enough and has moved, or something is malfuncitoning.

    Also, if mytiming was too advanced wouldn't this happend all the time? Not just when the engine was cold?

    Thanks for the description of engine a diesel...definitly not that sound, this sounded excactly like the very tip of my metal cooling fan was just barely tapping against something, about that speed and exactly that sound.

    Any other ideas?

  7. #7


    You could remove the fan belt. Start it up cold and drive it around the block. That would prove it one way or the other. Usually an engine will ping more when hot rather than cold. Another quick check is to loosen up the distributor and back it off about 1". That will retard the timing and it should remove the ping if in fact it is pinging.

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  8. #8


    Thanks Jimmie! Godd idea...of course since it is a serpentine belt removing it would mean no water pump or power steering? Is that a good idea?

    But that would for sure remove the fan from the suspect list.

  9. #9


    It wont hurt for a short trip.1 or 2 minutes with a cold engine. Now if you had a Ford it would spit out the PS fluid when turning but with a GM you should be OK. Just don't let it get hot!

  10. #10
    Sr. Apprentice
    Join Date
    Jul 2009
    Weatherford, TX


    Another way to check, without all the part removal, is to put super unleaded gas in your tank, or a bottle of octane booster, if you're usually running regular. If the pinging stops under the same circumstances, it could be a fuel delievery problem. or, it could just be bad gas in your area.

    i have an 85 carbureted, and an 89 TBI, and both HATE 87 gas from valero or conoco and some other cheapy stations. They will "ping" like crazy and i have to add a bottle of octane boost to make it go away.

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